New refine entry for the atan2 function

This PR adds the function refine_atan2. Compared with  PR #8476 in this one the functions is more clearly written with consisten variable names y, x. The doc string function is also corrected. 
I still don't understand. How this information can be used to reduce atan2 to atan. Can you provide an example of the expected output?.

In the solution proposed by @smichr the function _coeff_isneg is True in these two cases: _coeff_isneg(-n/m) and _coeff_isneg(n/-m) independently of whether m or n are positive or negative and with that is certainly not enough to define the quadrant and go from atan2 to atan.